Meh... PS1 games with half the resolution... I didn't like the portrait mode for games, especially for ports of games with landscape-aspect.

Gameboy Advance had a lot of of Tony Hawk's games... most of them with great reviews:

• https://www.metacritic.com/game/tony-hawks-pro-skater-2/critic-reviews/?platform=game-boy-advance
• https://www.metacritic.com/game/tony-hawks-pro-skater-3/critic-reviews/?platform=game-boy-advance
• https://www.metacritic.com/game/tony-hawks-pro-skater-4/critic-reviews/?platform=game-boy-advance

And just 1 year after the N-Gage (or 1.5 - 2 years depending on the region), the PSP happened with almost PS2 quality, better battery life... and you didn't have to removethe battery to change a game.

Including Tomb Raider Legend (2006), Tomb Raider Anniversary (2007) plus all the digital Tomb Raider games for PS1.

GameCube easily. I'd only rank it behind the Switch and it still beats it on some parameters.

The only noteworthy game N-Gage really had was Xanadu Next, which is now available on Steam. 

I'd hesitate to call Gamecube a "failed" system, considering it sold on par with Xbox and was profitable. It was just fashionable to shit on Nintendo at the time. Over the last couple of years, so many of my Gamecube favorites got ports on modern systems and I got most of them. Prime, Tales of Symphonia, Baton Kaitos, Story of Seasons: AWL. 

I don’t consider N64 to have been unsuccessful so I’d say Dreamcast. Great system, great games. Especially for the short time it was actually relevant. It ended up having a couple of my all-time favourites.

Dreamcast, imo. Hands down.

N64 would be it for me, but im unsure if id consider it "unsuccessful". Its seems that word is thrown around casually whenever a system isnt the biggest seller of its time.

Leaving the N64 aside, itd be the WiiU.

Sure I wouldn't call it a failure either, but "unsuccessful" seems applicable to me, particularly in Europe where I live. Damn shame since it had some of the greatest exclusives ever.

I always considered the N64, Xbox OG and Xbox One in that middle segment where you're not unsuccessful per day but you had problems that led you to being successful either.

Guess my choice would go on the GameCube or Dreamcast, since despite the few WiiU guys forget that it's software legacy has lived on the Switch gracefully.

GameCube: Pikmin 1+2, Zelda WW, SMSunshine, Star Fox Adventures+Assault, Luigi’s Mansion, Smash Melee.
DreamCast: Sonic Adventure 1+2, and a lot of other games I forget lol.